This invention relates to a method and system for coolant management in a polymer electrolyte membrane (PEM) fuel cell system. More particularly, the invention relates to the beneficial use of vacuum devices, and particularly an eductor, in the coolant flow circuit of the fuel cell system.
In the design and operation of fuel cell systems, and particularly fuel cell systems having a polymer electrolyte membrane (PEM), the management of the coolant, typically water, is challenging and important. The pressures, flow rates and volumes, and quality of the water in the coolant flow circuit of a PEM fuel cell system are critical to the continued, efficient operation of the system because the coolant system is the key to maintaining the removal of product water from the fuel cell stack, while also assuring that the membrane electrolyte does not dry out. The presence of incondensable (not readily condensable under normal operating conditions) gas in the coolant water creates water management problems that must be addressed. In various prior art fuel cell systems, water is isolated from reactant gases in the fuel stack, thus minimizing the entrainment/dissolution of gases into the coolant and the various accompanying considerations or limitations. In PEM fuel cell stacks employing water transfer plates (WTP) between adjacent fuel cells in the stack serving as the coolant distribution medium, the reactant gases present in the system come into intimate contact with the coolant water and are readily entrained and/or dissolved therein. Thus, the coolant management system must handle the circulation of fluids in two phases, or states, i.e., gaseous and liquid. This may be, and has been, done with volume flow devices, such as one or more positive displacement pumps. However, such pumps are relatively complex and expensive. Moreover, it is further desirable at some point to separate the entrained incondensable gases from the recirculating coolant. The removal of some gases, such as hydrogen (H2) and/or carbon dioxide (CO2), from the coolant is essential to prevent their concentration in the cooling system from building up. Conversely, some separation, or degasification, mechanisms may actually contribute to saturation of coolant with air.
Accordingly, it is an object of the invention to provide an improved coolant management system in a polymer electrolyte membrane fuel cell system. It is a further object to provide such coolant management system in a manner which is less complex and less expensive than prior systems.
The present invention comprises a method and system for managing water coolant in a PEM fuel cell system that includes water transport plates (WTPs). The invention comprises the use of a gas-liquid separating means having a vacuum device in a PEM fuel cell system of the type wherein incondensable gas is readily dissolved and/or entrained in circulating coolant, in part because of its contact with WTPs. The vacuum device in a preferred embodiment is a vacuum pump, such as an eductor (i.e., ejector), for transporting gas, or a gas-liquid mix, via suction. The gas-liquid separation involves at least the efficient transport of the gas, and preferably also the use of separator and/or accumulator means for furthering the gas-liquid separation and accumulating the liquid coolant. Further, a relatively simple and inexpensive coolant pump, such as a centrifugal pump or other similar dynamic pump, provides the circulatory motive force and driving pressure for the coolant water in the coolant system.
In addition to anode and cathode reactant channels in and for a fuel cell assembly (CSA), the CSA further includes a coolant channel, or cooler, containing the water transfer plates (WTP""s). Gas, such as air, hydrogen, carbon dioxide, etc., entrained and/or dissolved in the liquid coolant as it flows past the WTP""s, is removed from the coolant with the assistance of a vacuum device, such as an eductor, and additional separator/accumulator means. The term xe2x80x9cgasxe2x80x9d, as used herein, is intended to mean a normally incondensable gas entrained and/or dissolved in the coolant, as contrasted with steam, which is condensable. The separator/accumulator means may be, for instance, a cyclone separator/accumulator and/or a bubble trap separator/accumulator.
The eductor includes a primary, or motive, inlet, a secondary, or suction, inlet and a discharge exit, or outlet. Coolant water from the coolant pump is supplied to the eductor""s motive inlet. The eductor""s suction inlet is connected to a section of the coolant circuit designed to allow relative separation of the entrained gases, such that the vacuum draws at least the gaseous portion to, and through, the eductor. A separator/accumulator receives the effluent from the eductor for further gas separation and accumulation of the liquid coolant for return to the coolant circuit.
In one embodiment, a gas/liquid mix coolant is drawn by vacuum through the eductor, and then subsequently separated. Accumulated liquid coolant is then supplied to the coolant pump. In another embodiment, a preliminary separation of gas from liquid involves a bubble trap separator, or the like, so that the eductor draws primarily gas from the trap. The remaining liquid coolant is supplied first to the coolant pump and then the flow stream is split between the eductor""s motive inlet and the further separator/accumulator before return to the coolant circuit. The separated gas, i.e., air, hydrogen, carbon dioxide, etc., may be vented from the system or, in the instance of air, returned for use as oxidant reactant at the cathode of the CSA. A demineralizer may be connected in shunt feedback relation with the coolant pump to maintain the desired water quality.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.